Hydrometallurgical processes are commonly used to obtain valuable base metals as well as other metals from ores and tailings containing these metals. Several major operations are typically involved in such processes. Initially, a leaching of the crushed ore or tailings is performed using a lixiviant (e.g. using an aqueous acidic solution such as sulphuric acid solution) to dissolve the metal content from crushed ore or tailings and provide a leach solution comprising the desired metal or metals. The leach solution of course generally also comprises unwanted liquid and solid impurities which are desirably removed. Further, the leach solution is generally quite dilute. Thus, solution concentration and purification operations are performed to produce a more concentrated, purified solution known in the art as pregnant solution stream. Finally then, operations are performed to recover the desired metals from the pregnant solution stream.
A variety of lixiviants may be considered for such hydrometallurgical processing and the selection depends on the metal to be extracted and the type of ore or tailings involved. Aqueous acid solutions are perhaps most commonly employed as lixiviants and include solutions of such acids as sulfuric acid, hydrochloric acid, and nitric acid. Further, various arrangements are commonly considered in order to accomplish the leaching process including heap leaching, dump leaching, tank leaching and in-situ leaching arrangements.
The chemistries and equipment involved in the solution concentration and purification operations can also be quite diverse and depend on the metals and ores or tailings involved. However, a solvent extraction step is frequently employed in which an organic solvent is used to selectively extract the desired metal content from the impure leach solution to the organic phase. This is followed by a stripping of the desired metal content from the organic phase using strong acid to produce the pregnant solution stream which is a strong electrolyte or concentrated metal solution. The solvent extraction step also results in a raffinate stream or waste stream which is an aqueous solution containing low metal content with appreciable amount of residual acidity and traces of organics. The raffinate stream is commonly divided into two portions in which one portion is recycled to the leaching circuit and the other is sent to neutralization where it is treated with lime to neutralize the acidity and to precipitate the metal contents prior to discharge as effluent in order to meet environmental regulatory requirements.
The metal recovery operations separate the desired metals from the concentrated pregnant solution stream. Typical metal recovery processes include electrolytic processing (e.g. “electrowinning” or “electrorefining”), chemical precipitation, and gaseous reduction. In operations involving electrolytic processing, the pregnant solution stream serves as electrolyte in the associated electrolysis. There is generally a need to continuously bleed a proportion of the tankhouse inventory however, as “spent electrolyte”, in order to prevent excess build-up of contaminants transferred from the primary leach solution and to maintain the purity of the recovered metal product. This bleed stream is commonly “disposed of” in several ways: (i) is added to the primary feed of the solvent extracting circuit where the desired metal is recovered and sent around again to the electrolytic operation; (ii) is taken directly back to the leaching circuit and cycles back through an even longer path; and/or (iii) is sent as a purge stream for metal recovery and acid neutralization prior to disposal as effluent.
There are therefore two major effluent streams that are typically generated in hydrometallurgical processes that need to be managed properly. These are the spent electrolyte stream in processes involving electrolytic processing and the raffinate stream in processes involving solvent extraction. While the common chemical (lime) treatment of the raffinate stream meets environmental standards, it unfortunately involves a series of complex precipitation and separation steps which are not only expensive to set up and operate but also which generate a large amount of gypsum solids that are often classified as hazardous materials due to its heavy metal contents and thus must be properly disposed of. Further, the acid content in the raffinate is lost and yet this is preferably recovered instead. As for spent electrolyte streams, recycling them in whole or in great part to the leaching circuit would likely overload the circuit and negatively impact the overall leaching efficiency due to its high acid and metal concentration. Further, sending them in the whole or great part to the stripping operation in a solvent extracting circuit would also affect the stripping efficiency as well as increase the capacity requirement of the overall solvent extracting circuit. And purging a side stream of spent electrolyte to metals recovery and neutralization processes would increase acid losses as well as lime consumption. There is therefore a need to better handle such effluent streams in hydrometallurgical processes. The present invention addresses this need and provides other benefits as disclosed below.
Nanofiltration is an example of a pressure driven membrane separation process in which organic molecules or inorganic ionic solutes in aqueous solutions are concentrated or separated to various degrees by the application of a positive osmotic pressure to one side of a filtration membrane. Such a pressure driven membrane process employs a cross-flow mode of operation in which only a portion of a feed stream solution is collected as a permeate solution and the rest is collected as a retentate solution. Thus, in a nanofiltration module, the exiting process stream which has not passed through the nanofiltration membrane is referred to as the “retentate” stream and the exiting process stream which has passed through the membrane is referred to as the “permeate” stream.
NF membranes reject ionic species according to the charge density of the ions and the surface charges of the membrane. Accordingly, divalent anions, such as SO42−, are more strongly rejected than monovalent ones, such as Cl−, and divalent cations, such as Cu2+, are more strongly rejected than monovalent ones, such as Na+. And therefore, nanofiltration can be particularly suitable for processes requiring separation of multivalent anions from monovalent ones.
Commercial NF membranes are available from known suppliers of reverse osmosis and other pressure driven membranes. The NF membranes are, typically, packaged as membrane modules. A so-called “spiral wound” module is most popular, but other membrane module configurations, such as tubular membranes enclosed in a shell or plate- and -frame type, are also known.
During the NF process, a minimum pressure equal to the osmotic pressure difference between the feed/retentate liquor on one side and the permeate liquor on the other side of the membrane must be applied since osmotic pressure is a function of the ionic strengths of the two streams. In the case of separation of a multivalent solute, e.g. Na2SO4, from a monovalent one, e.g. NaCl, the osmotic pressure difference is moderated by the low NaCl rejection. Usually, a pressure in excess of the osmotic pressure difference is employed to achieve practical permeate flux.